Modernize omnibus cookbook

:construction: :red_circle: WIP

- [ ] Add ChefSpec coverage
- [x] Replace rbenv with chruby/ruby-install
- [x] Update to latest cookbook versions
- [ ] Install bash

Note: I talked with @lamont-granquist about this. We will need to install `bash` on some systems (like freebsd/aix).

/cc @opscode-cookbooks/release-engineers